After a borehole has penetrated a formation and a casing has been cemented in place, the hydrocarbon-containing formation must be communicated with the wellhead so that valuable hydrocarbons can be extracted from the wellbore. Roy R. Vann U.S. Pat. Nos. 3,706,344 and 3,871,448 teach a permanent completion technique which can advantageously be employed in completing a borehole. Reference is made to these prior patents, to Patent Nos. 3,931,855; 3,812,911; and 4,040,485; and to the art cited therein for further background of the present invention.
The well completion method and apparatus of the present invention is particularly applicable to deep, high-temperature, high-pressure wells. For example, such a well might be over 10,000 feet deep, have a bottomhole temperature of about 300.degree. F., and bottomhole pressure of over 5,000 psi. Because of this environment, it is essential for safety reasons that control be maintained over the well at all times. Such control is maintained by using a hydrostatic head of well fluids such as mud to insure that the bottomhole pressure exceeds the formation pressure and later setting a packer in the cased borehole.
The hydrostatic head of drilling mud is maintained in the well to control the high-pressure production zone. The object is to prevent a blowout of the well. Any replacement of the mud with a lighter clean fluid prior to the setting of the packer removes the margin of safety. Thus, it is desirable to keep heavy mud in the well as long as possible to insure that the production zone is killed even if there is a casing leak, for example. Once the packer has been set and the cased borehole can be sealed off, the margin of safety can be maintained. The hydrostatic head is still maintained in the annulus above the packer. A permanent packer is almost always used in a deep, hot, high-pressure well. A permanent packer will contain and withstand the temperatures and pressures of such a well since a permanent packer is a more heavy-duty packer. Thus, very few customers will permit a retrievable packer in such a well where the retrievable packer is mounted on a tubing string with a perforating gun. Such a packer is considered not strong enough to insure the control of the differential pressures and high temperatures in the well. For example, such a packer might fail after the mud was removed with a lighter fluid in the tubing string or where the tubing string is swabbed dry in preparation for perforation. A retrievable packer, mounted on a tubing string with a perforating gun, is generally inadequate for a well with this environment.
The tool string with permanent packer, perforating gun, and other associated apparatus may be run into the well either on a wireline or on a tubing string. It is much more convenient to run the permanent packer in on a wireline rather than a tubing string, however, since less rig time is used. If a tubing string is used, one must come back out of the hole after the packer is set. Once out of the hole, the packer setting tool is taken off the tubing string and then a sealing nipple is run back into the well. This procedure adds almost another day to the well completion.
Although the wireline is preferred, the tool string may have to be lowered on a tubing string where the tool string is very heavy, as for example, with a heavy, long perforating gun or a string of perforating guns. In using a tubing string to lower and set the permanent packer and tool string, the tubing string is released from the permanent packer and removed from the well after the packer has been set.
When a permanent packer, such as described, is required because of the well environment, the tool string is not a closed system, i.e., such as with a vent assembly that keeps mud out of the string. There must be a flow path for circulation to remove the mud. Such a flow path must be available upon running the packer into the hole. Thus, the tool string on the permanent packer completely fills up with drilling mud as it is run into the hole and left in the hole for a long period of time prior to completing the well. The tool string is completely surrounded with drilling mud from the mud in the well.
Thus, it is often desirable to be able to carry out the necessary steps to suspend a perforating gun from a permanent packer device while the drilling rig is on location, and to complete the well at some subsequent time. There are also advantages in running in and setting the permanent packer with tool string in the well and leaving the tool string downhole for several days before completing the well. It may be desirable to leave the downhole dormant for a period of time. For example, one may wish to leave the downhole dormant until one is prepared to complete several wells and tie them all in at one time. However, when a tool is left downhole in a borehole, the surrounding mud often contaminates the interior of the tool. This is because the heavy particles of the mud and other suspended matter gravitate toward the bottom of the tool string where the contaminant densifies into a heavy layer of material. The longer the tool string is left downhole, the more the drilling mud is permitted to settle and congeal. In a perforating gun having a bar actuated gun firing head, for example, it is possible for the mud to densify about the gun firing head mechanism and become compacted and viscous to such an extent that the gun firing head cannot be impacted and detonated. The firing mechanism will require 20 ft-lbs of impact for detonation. Where the mud is permitted to settle and congeal, it may well be impossible to attain 20 ft-lbs of impact from the bar. Thus, it is an advantage to prevent the mud and debris from collecting around the firing mechanism and to permit circulation for the removal of any packed mud. Further, it is advantageous to be able to isolate all of the tubing string above the gun and yet easily gain access to the gun firing head so that no contamination thereof can occur. Method and apparatus for accomplishing this purpose is the subject of the present invention.